In recent years there has been an increased concern in the United States and elsewhere about air pollution from industrial emissions of noxious oxides of nitrogen, sulfur and carbon. In response to such concerns, government agencies have in some cases already placed limits on allowable emissions of one or more of the pollutants, and the trend is clearly in the direction of increasingly stringent restrictions.
NOx, or oxides of nitrogen, in flue gas streams exiting from fluid catalytic cracking (FCC) regenerators is a pervasive problem. Fluid catalytic cracking units (FCCU) process heavy hydrocarbon feeds containing nitrogen compounds a portion of which is contained in the coke on the catalyst as it enters the regenerator. Some of this coke nitrogen is eventually converted into NOx emissions, either in the FCC regenerator or in a downstream CO boiler. Thus all FCCUs processing nitrogen-containing feeds can have a NOx emissions problem due to catalyst regeneration.
In an FCC process, catalyst particles (inventory) are repeatedly circulated between a catalytic cracking zone and a catalyst regeneration zone. During regeneration, coke deposited from the cracking reaction on the catalyst particles is removed at elevated temperatures by oxidation with oxygen containing gases such as air. The removal of coke deposits restores the activity of the catalyst particles to the point where they can be reused in the cracking reaction. The coke removal step is performed over a wide range of oxygen availability conditions. At the minimum, there is typically at least enough oxygen to convert all the coke made to CO and H2O. At the maximum, the amount of oxygen available is equal to or greater than the amount necessary to oxidize all the coke to CO2 and H2O.
In an FCC unit operating with sufficient air to convert essentially all of the coke on the catalyst to CO2 and H2O, the gas effluent exiting the regenerator will contain “excess oxygen” (typically 0.5 to 4% of total off gas). This combustion mode of operation is usually called “full burn”. When the fluid catalytic cracking unit (FCCU) regenerator is operating in full burn mode, the conditions in the regenerator are for the most part oxidizing. That is, there is at least enough oxygen to convert (burn) all reducing gas phase species (e.g., CO, ammonia, HCN) regardless of whether this actually happens during the residence time of these species in the regenerator. Under these conditions, essentially all of the nitrogen deposited with coke on the catalyst during the cracking process in the FCCU riser is eventually converted to molecular nitrogen or NOx and exits the regenerator as such with the off gas. The amount of coke nitrogen converted to NOx as opposed to molecular nitrogen depends on the design, conditions and operation of the FCCU and especially of the regenerator, but typically the majority of coke nitrogen exits the regenerator as molecular nitrogen.
On the other hand, when the amount of air added to the FCCU regenerator is insufficient to fully oxidize the coke on the cracking catalyst to CO2 and H2O, some of the coke remains on the catalyst, while a significant portion of the coke carbon burned is oxidized only to CO. In FCCUs operating in this fashion, oxygen may or may not be present in the regenerator off gas. However, should any oxygen be present in the regenerator off gas, it is typically not enough to convert all of the CO in a gas stream to CO2 according to the chemical stoichiometry ofCO+½O2→CO2 This mode of operation is usually called “partial burn.” When an FCCU regenerator is operating in partial burn mode, the CO produced, a known pollutant, cannot be discharged untreated to the atmosphere. To remove the CO from the regenerator off gas and realize the benefits of recovering the heat associated with burning it, refiners typically burn the CO in the regenerator off gas with the assistance of added fuel and air in a burner usually referred to as “the CO boiler”. The heat recovered by burning the CO is used to generate steam.
When the regenerator is operating in partial burn, the conditions in the regenerator, where the oxygen added with air has been depleted and CO concentration has built up, are overall reducing. That is, there is not enough oxygen to convert/burn all reducing species regardless if some oxygen is actually still present. Under these conditions some of the coke-nitrogen is converted to so called “gas phase reduced nitrogen species”, examples of which are ammonia and HCN. NOx may sometimes also be present in the partial burn regenerator off gas, but typically only in small amounts. When these gas phase reduced nitrogen species are burnt in the CO boiler with the rest of the regenerator off gas, they can be oxidized to NOx, which is then emitted to the atmosphere. This NOx along with any “thermal” NOx formed in the CO boiler burner by oxidizing atmospheric N2 constitute the total NOx emissions of the FCCU unit operating in a partial or incomplete combustion mode.
FCCU regenerators may also be designed and operated in a “incomplete burn” mode intermediate between full burn and partial burn modes. An example of such an intermediate regime occurs when enough CO is generated in the FCCU regenerator to require the use of a CO boiler, but because the amounts of air added are large enough to bring the unit close to full burn operation mode, significant amounts of oxygen can be found in the off gas and large sections of the regenerator are actually operating under oxidizing conditions. In such case, while gas phase reduced nitrogen species can still be found in the off gas, significant amounts of NOx are also present. In most cases a majority of this NOx is not converted in the CO boiler and ends up being emitted to the atmosphere.
Yet another combustion mode of operating an FCCU is nominally in full burn with relatively low amounts of excess oxygen and/or inefficient mixing of air with coked catalyst. In this case, large sections of the regenerator may be under reducing conditions even if the overall regenerator is nominally oxidizing. Under these conditions reduced nitrogen species may be found in the regenerator off gas along with NOx.
Simultaneously with NOx emissions, afterburn may also be a concern for units operating in partial burn or incomplete combustion mode. Gases exiting the catalyst bed of an FCCU operating in partial burn combustion mode will consist mainly of CO2, CO, H2O, reduced nitrogen gases, other reduced species such as H2S and COS, SO2, and potentially some O2 and/or NO. However, depending on the design and mechanical condition of the regenerator, conditions can develop in which sufficient amounts of CO and O2 escape the catalyst bed allowing the CO to react with the available O2. The reaction can occur in the regenerator at any point above the dense catalyst bed, including the area above the dense bed (dilute phase), the cyclones where entrained catalyst is separated from the flue gas, the plenum above the cyclones, or even the flue gas pipe. Because afterburn occurs after the dense bed of the cracking catalyst, which acts as a heat sink absorbing the heat released from the exothermic reaction of CO with O2, it can heat up the gases to the point that overheating can occur. The result can be temperatures which approach the metallurgical limit of the materials used to construct the regenerator. High afterburn can limit the useful life of the regenerator equipment, and runaway afterburn can cause catastrophic equipment failure.
Typically, afterburn is prevented or controlled by adding CO combustion promoters to the cracking catalyst circulating inventory which promote the combustion of CO to CO2. Conventional CO combustion promoters typically comprise an additive comprising 300 to 1000 ppm platinum or alumina, or a much smaller amount of platinum, e.g., amounts which typically achieve from about 0.1 to about 10 ppm in the total cracking catalyst inventory, incorporated directly into the cracking catalyst itself.
While CO combustion promoters can be effectively used to prevent or control afterburn in FCC units operated in partial burn or incomplete combustion mode, the use of combustion promoters is not desirable in many FCC units. By promoting the reaction of CO to CO2 in an oxygen deficient environment, a combustion promoter can consume oxygen to convert CO, oxygen which otherwise would have been used to convert coke to CO, thereby increasing coke left on the regenerated catalyst (CRC). Increased amounts of CRC on the cracking catalyst returned to the riser will decrease the catalyst activity, and may reduce conversion and product yields. Any increase in the conversion of CO will also increase the heat released in the regenerator and as a result can raise the temperature in the dense catalyst bed. Increasing the dense bed temperature can often be undesirable, since higher regenerated catalyst temperature can negatively affect catalyst circulation, catalyst activity and stability, unit conversion and/or product yields. Thus, many FCC units operated in partial burn or incomplete combustion mode, can not use any CO combustion promoter or any other additive having CO oxidation activity sufficient to be useful as a CO oxidation promoter under catalytic cracking conditions.
Various catalytic approaches have been proposed to control NOx emissions in the flue gas exiting from the FCCU regenerator.
For example, recent patents, including U.S. Pat. Nos. 6,379,536, 6,280,607, 6,129,834 and 6,143,167, have proposed the use of NOx removal compositions for reducing NOx emissions from an FCCU regenerator. U.S. Pat. Nos. 6,358,881 and 6,165,933 also disclose a NOx reduction composition which promotes CO combustion during an FCC catalyst regeneration process step while simultaneously reducing the level of NOx emitted during the regeneration step. NOx compositions disclosed by these patents may be used as an additive, which is circulated along with the FCC catalyst inventory or incorporated as an integral part of the FCC catalyst.
In U.S. Pat. No. 4,290,878, NOx, is controlled in the presence of platinum-promoted CO oxidative promoters in a full burn combustion regenerator by the addition of iridium or rhodium on the combustion promoter in amounts lesser than the amount of Pt.
U.S. Pat. Nos. 4,973,399 and 4,980,052 disclose copper-loaded zeolite additives useful for reducing emissions of NOx from the regenerator of an FCCU unit operating in full burn combustion mode.
U.S. Pat. No. 4,368,057, teaches the removal of NH3 contaminants of gaseous fuel by reacting the NH3 with a sufficient amount of NOx.
However, aforementioned prior art has failed to appreciate an FCC process which minimizes the amount of gas phase reduced nitrogen species, e.g. NH3 and HCN, in the flue gas of an FCCU regenerator operating in a partial or incomplete combustion mode.
Efforts to control ammonia released in an FCC regenerator operated in a partial or an incomplete mode of combustion have been known.
For example, U.S. Pat. No. 5,021,144 discloses reducing ammonia in an FCC regenerator operating in a partial burn combustion mode by adding a significant excess of the amount of a carbon monoxide (CO) combustion promoter sufficient to prevent afterburn in the dilute phase of the regenerator.
U.S. Pat. No. 4,755,282 discloses a process for reducing the content of ammonia in a regeneration zone off gas of an FCCU regenerator operating in a partial or incomplete combustion mode. The process requires passing a fine sized, i.e. 10 to 40 microns, ammonia decomposition catalyst to either the regeneration zone of an FCCU, or an admixture with the off gas from the regeneration zone of the FCCU, at a predetermined make-up rate such that the residence time of the decomposition catalyst relative to the larger FCC catalyst particles will be short in the dense bed of the regenerator due to rapid elutriation of the fine sized ammonia decomposition catalyst particles. The fine sized elutriated decomposition catalyst particles are captured by a third stage cyclone separator and recycled to the regenerator of the FCCU. The decomposition catalyst may be a noble group metal dispersed on an inorganic support.
U.S. Pat. No. 4,744,962 is illustrative of a post-treatment process to reduce ammonia in the FCCU regenerator flue gas. The post-treatment involves treating the regenerator flue gas to lessen the ammonia content after the gas has exited the FCCU regenerator but before passage to the CO boiler.
There remains a need in the refining industry for improved FCC processes which minimizes the content of gas phase reduced nitrogen species and NOx emitted from a partial or incomplete combustion FCCU regenerator which processes are simple and do not require additional equipment, time and expense typically associated with prior FCC processes for the removal of the gas phase reduced nitrogen species in the regenerator off gas.